Fatigue damage study of cold metal transition fusion-brazed aluminium/steel dissimilar joints

ABSTRACT

An aluminium/steel CMT-welded joint is mainly composed of fusion zone (FZ), interfacial intermetallic compound (IMC) zone, and heat-affected zone (HAZ). IMC and FZ are the weakest link in the welded joint. The fatigue fracture indicates that the lattice distortion caused by the lattice mismatch of Fe₂Al₅ and Fe is more likely to engender dislocation slipping and cracks easily nucleate during the fatigue testing. The hysteresis loop is shifted to the right, which indicating that the specimen experiences a ratcheting effect. In the high-stress condition, the cyclic hardening induced by the ratcheting strain is much stronger than that in the lower stress condition, which improves the fatigue life of the materials.     

覆岩结构类型与开采覆岩损害特征分析

煤层开采打破了上覆岩层原有的平衡,必然引起岩层的移动、破断,导致应力场和裂隙场的改变,工作面出现矿压显现。上覆岩层的岩性和组合结构对工作面开采覆岩损害特征分布影响显著,一般岩性越坚硬,上覆岩层应力集中程度越高,采动裂隙越发育,矿压显现也越明显;坚硬岩层尤其是关键层对覆岩的破断运动起控制作用,覆岩破断发展到一定层位后软弱岩层往往是抑制覆岩破断进一步向上发展的关键。当工作面上覆岩层同时存在远近场关键层时,近场关键层的周期性破断引起裂隙带范围内岩层的运动和工作面周期来压;而远场关键层的周期性破断导致地表裂缝或台阶下沉,工作面压力急剧上升,有时甚至导致煤矿动压灾害,尤其是远近场关键层同步破断时动力现象更明显。通过地面钻孔使远场关键层提前破断是防止此类动力灾害的关键。     

转速对2524铝合金搅拌摩擦点焊组织与性能的影响

对2524铝合金薄板进行搭接搅拌摩擦点焊试验, 研究了搅拌头转速对其组织与性能的影响。结果表明, 随转速增加, 焊点外侧飞边越加严重, 组织畸变程度增加, 焊核区宽度增大, 有效连接宽度先增大后减小。母材区硬度最大, 热影响区硬度最低;在焊核区内, 显微硬度随转速增加而增大, 当转速为1 100 r/min时达到最大133.9HV, 而热影响区在300 r/min时硬度最大。在单向拉伸试验中, 接头剪切强度随转速增加先增大后减小, 在700 r/min时达到最大值3 510 N;接头呈现出两种断裂模式: 即低转速下平行于两板面断裂和高转速下沿倾斜方向断裂;断口呈现明显的剪切韧窝形态, 韧窝小而浅, 焊接接头为韧性断裂。     

铸态Ti-20Zr-20Al钛合金动态压缩力学性能与断裂机制研究

采用X射线衍射仪、金相显微镜、分离式霍普金森杆和扫描电镜等手段研究了Ti-20Zr-20Al钛合金在动态压缩条件下的微观结构、力学性能和断裂机制。结果表明,Ti-20Zr-20Al铸态合金由Zr基体相和TiAl/Ti₃Al针状相组成; 随着应变率增加,合金的抗压强度增加,失效应变显著增加; Ti-20Zr-20Al合金的断裂机制为解理断裂; 随着应变率增加,试验压缩断裂后碎片总数增大,平均粒径减小。采用DID模型模拟的材料碎片尺度与实验结果比较吻合,Grady模型与实验结果的偏差较大。     

Effects of microstructure on fracture strength and conductivity of sintered NMC333

Sintering of LiNi_0.33Mn_0.33Co_0.33O₂ cathode material was investigated for potential application in all-electric aerospace propulsion systems utilizing new architectural concepts. All-solid-state batteries, while inherently safe, may not reach the high energy density required for next generation propulsion systems. To meet this performance requirement, multifunctionality of sintered active material may achieve systems level weight savings through simultaneous load bearing and electrochemical energy storage performance. The effects of sintering conditions on structural stability, chemistry, densification, grain size, fracture strength and electrical conductivity were quantified for the active cathode material. X-ray diffraction and inductively coupled plasma results indicated the structure and stoichiometry were maintained across the range of processing conditions to facilitate intercalation. Densification was achieved by sintering at 1050°C in ambient atmosphere, but grain coarsening was observed for higher temperatures and longer hold times. Mechanical strength was improved with reduction in porosity, but excessive grain growth decreased strength, providing a maximum of 50 MPa for samples sintered at 1050°C for 10 hours. Electrical conductivity initially improved with densification, but significantly diminished as the microstructure coarsened. The optimal sintering condition of 1050°C maximized mechanical fracture strength and electrical conductivity, with shorter sintering times preferred.     

电磁精细探测法在隐伏型导水地质裂缝勘探中的应用

针对传统隐伏型导水地质裂缝勘探方法存在勘测精度较差的问题,提出电磁精细探测法探析隐伏型导水地质裂缝。依照屏蔽系数、实测场强和理论场强数据绘制综合曲线图,通过该图获取隐伏型导水地质裂缝所处位置几何阴影范围,采用层析成像法得到网格化的工作面,获取隐伏型导水地质图像。通过图像直接观测隐伏型导水地质工作面裂缝所处位置,在此基础上,观测四个电磁场分量,采用正交电磁场分量计算介质视电阻率,依据计算视电阻率数值和视电阻率分布状态研究裂缝发育情况和裂缝富水程度。结果表明:采用该方法能较为精准地获取隐伏型导水地质裂缝位置。通过裂缝位置进一步检测出隐伏型导水地质裂缝最大发育高度为63.5 m。当视电阻数值不断增加时,隐伏型导水地质裂缝和裂缝富水性逐渐减小,与实际情况较为相符,说明该种方法探析效果较好。     

大采高工作面过断层破碎带围岩控制技术研究

为了研究大采高工作面断层破碎带围岩控制技术,采用UDEC分别模拟工作面揭露断层前后围岩应力变化规律、揭露断层时覆岩运移规律。根据模拟结果可知,断层导致工作面应力呈"几"字型分布及顶底板变形量增大。在断层影响区域采用锚网索、单体液压支柱挑棚联合支护取得了良好的效果。     

Development and in vivo response of hydroxyapatite/whitlockite from chicken bones as bone substitute using a chitosan membrane for guided bone regeneration

Biomaterials that meet the requirements to stimulate bone tissue formation play a vital role in orthopedics and dentistry. In this work, chitosan and a biphasic, non-cytotoxic material hydroxyapatite/whitlockite were obtained from natural sources, which are available as organic waste. The osteogenic activity was assessed using a rabbit model animal with a chitosan barrier membrane in combination with a bone-filling graft substitute composed of hydroxyapatite/whitlockite. FT-IR results showed the typical absorption bands of the chitosan and hydroxyapatite. Moreover, the X-ray diffraction pattern revealed a typical hexagonal phase of hydroxyapatite and rhombohedral structures related to whitlockite. Masson's trichrome stain showed an early formation of extracellular matrix mineralized, in accord with the surface morphology of a cortical mature bond observed by Scanning Electron Microscopy. The immunocytochemistry results showed a significant increase of positive immunoreactive cells to osteonectin in the treated defects in comparison with the control defects 6 and 8 weeks postoperatively. Overall, the results confirm that the use of this low-cost and versatile biomaterial as a barrier membrane and a bone substitute graft are useful for bone tissue engineering.     

Assessment of the effect of residual stresses in elastic-plastic fracture of dissimilar welded components

Residual stresses in welds pose a significant threat to the structural integrity of a component, especially in the presence of defects and are required to be accounted for in assessing component safety. Although the R6 assessment procedure suggests various approximate methods for incorporating these effects in defect assessment, most of them are overly conservative and not very cost-effective. A more reliable approach is to characterise the weld residual stresses around a defect and study how they interact with primary load. The current paper analyses the effects of weld residual stresses on the fracture of a dissimilar weld in the presence of defect. The weld is made between modified 9Cr–1Mo steel and 316LN stainless steel using autogenous electron beam welding. A C(T) specimen was extracted from the centre of the weld and a crack introduced in the fusion zone using electro-discharge machining. The residual stresses around the crack were measured on a grid of measurement points at mid-thickness of the C(T) specimen using neutron diffraction on the strain diffractometer SALSA at ILL, Grenoble. The measured residual stresses around the crack-tip were incorporated into a finite element model and the interaction of these with applied load was predicted under fracture.     

Shear crack growth in brittle materials modeled by constrained Cosserat elasticity

The propagation of in-plane shear cracks is investigated in brittle microstructured materials modeled by the constrained Cosserat elasticity. This theory introduces characteristic material lengths in order to describe the scale effects that emerge from the underlying microstructure and has proved to be very effective for modeling complex microstructured materials. An exact solution is obtained based on integral transforms and the Wiener-Hopf technique. Numerical results are presented illustrating the dependence of the stress intensity factor and the energy release rate upon the loading profile, the propagation velocity, and the characteristic material lengths of Cosserat elasticity. It is shown that depending on the Cosserat microstructural lengths the limiting crack propagation velocity can be significantly lower than the classical Rayleigh limit. Moreover, strengthening effects are observed when the characteristic material lengths become comparable to the geometrical lengths of the problem, a behavior that has been experimentally verified in fracture of ceramics.